Hydraulic fracturing of subterranean formations is an old and highly developed process, used primarily to increase the permeability of a portion of a formation surrounding a wellbore. The process may be applied to new wells to increase productivity, or to old wells to increase or restore productivity. The process is also applicable to injection wells used in secondary recovery or fluid disposal operations.
In a typical fracturing process, a thickened fluid such as an aqueous gel or an emulsion is utilized. The thickened fluid increases the fracturing effect and also supports proppant material which is deposited in the fractures created by the process. In many cases, a fluid loss additive material is included with the fracturing fluid to further enhance the results. A common fluid loss additive material is silica flour. Many other natural and synthetic solid materials have been utilized as fluid loss additives in fracturing processes.
A detailed description of the hydraulic fracturing process, including a recitation of suitable gelling agents useful therein, is found in U.S. Pat. No. 4,470,915 to Conway.
When solid fluid loss additives are included in the fracturing fluid, a gel filter pad comprising fluid loss additive and concentrated gel material forms on the surfaces of the wellbore and the fractures creates by the process. Ideally, this gel filter pad is subsequently removed by backflow of fluid from the formation (except in the case of injection wells), but in actual practice, it is usually necessary to follow the treatment with gel breaking and/or gel filter pad removal steps. These steps often only recover a small fraction of the potential productivity of the well.
A fracturing fluid comprised of an aqueous gel and a hydrolyzable organic ester which breaks the gel is described in U.S. Pat. No. 3,960,736 to Oree et al. That patent does not suggest that the organic ester is used in an amount or form to provide fluid loss properties to the treatment fluid.
U.S. Pat. Nos. 4,387,769 and 4,526,695 to Erbstoesser et al. describe use of a polyester polymer as a fluid loss additive material. The polymers degrade at formation conditions to facilitate removal from the treated well.
U.S. Pat. No. 3,868,998 to Lybarger et al. describes a process for placing a self-cleaning pack of particles in a formation utilizing a solution of a slowly reactive acid-yielding material.
U.S. Pat. No. 4,715,967 to Bellis et al. describes a condensation product of hydroxyacetic acid with itself or other compounds, the condensation product having the ability to provide fluid loss properties to a fluid and being degradable at formation conditions. The condensation products described in that patent are particularly useful in the process of this invention.
According to the present invention, condensation products of the type described in the aforementioned Bellis et al. patent are utilized in a fracturing fluid to provide fluid loss properties and to also provide gel breaking capabilities such that the gel filter pad comprised of condensation product and concentrated gel on the wellbore and fracture surfaces is essentially completely removed, thereby restoring full permeability to the well.